Autism, one of the best validated neuropsychiatric disorders of childhood, may represent the final common outcome of multiple causative factors. Heredity appears to play an important role: families with one autistic child face a recurrence risk for subsequent children that is 100 times greater than the population risk. Recent studies of twins show that between 60 and 90% of identical twin pairs are concordant for autism, whereas in non-identical twins, the concordance rate is zero. Together, these data strongly support the hypothesis that autism results from a strong, thought not absolute, genetic influence. The Utah Autism Program Project aims to identify genetic and non-genetic subgroups of autism by studying possible manifestations of the disorder and the familiar amount of transmission of these manifestations in autistic children and their first-degree family members. The findings will be contrasted to control groups of families in which a proband has been affected with a nonspecific mental retardation or with dyslexia. Features to be measured include immunologic competence, cognitive, psychiatric, morphologic differences and finally, variables in the brain detected by magnetic resonance imaging studies.